Magnetic random access memory (MRAM) is a type of non-volatile magnetic memory which includes magnetic memory cells. A typical magnetic memory cell includes a layer of magnetic film in which the magnetization of the magnetic film is alterable and a layer of magnetic film in which magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization is typically referred to as a data storage layer, and the magnetic film which is pinned is typically referred to as a reference layer.
A typical magnetic memory includes an array of magnetic memory cells. Word lines extend along rows of the magnetic memory cells, and bit lines extend along columns of the magnetic memory cells. Each magnetic memory cell is located at an intersection of a word line and a bit line. A magnetic memory cell is usually written to a desired logic state by applying external magnetic fields that rotate the orientation of magnetization in its data storage layer. The logic state of a magnetic memory cell is indicated by its resistance which depends on the relative orientations of magnetization in its data storage and reference layers. The magnetization orientation of the magnetic memory cell assumes one of two stable orientations at any given time. These two stable orientations are referred to as “parallel” and “anti-parallel” orientations. With parallel orientation, the orientation of magnetization in the data storage layer is substantially parallel to the magnetization in the reference layer along the easy axis and the magnetic memory cell is in a low resistance state which can be represented by the value R. With anti-parallel orientation, the orientation of magnetization in the data storage layer is substantially anti-parallel to the magnetization in the reference layer along the easy axis and the magnetic memory cell is in a high resistance state which can be represented by the value R+ΔR. A sense amplifier can be used to sense the resistance state of a selected magnetic memory cell to determine the logic state stored in the memory cell.
The ability of the sense amplifiers to quickly and accurately sense the values of R and R+ΔR depends on the physical design of the sense amplifier and can be affected by such factors as transistor thresholds, process variations, the mismatching of device sizes, and operating conditions which include power supply voltage and ambient temperature. Variations in these factors can result in offset error in the sense amplifiers which can reduce their speed and accuracy. If these variations are significant, data stored in the magnetic memory can become unreliable.
Calibration of the sense amplifiers is typically performed only once when the magnetic memory is first powered up. With this approach, once the sense amplifiers are calibrated, no further calibration is performed. Because the power supply voltage or ambient temperature of the magnetic memory can change after the magnetic memory is powered up, this approach can result in decreased reliability and performance.